The purpose of this proposal is to define the mechanisms of Notch1 signaling in both in vitro and in vivo systems. Notch1 is an evolutionarily conserved transmembrane receptor, important in mediating protection from apoptosis in developing thymocytes and in influencing CD4 vs CD8 lineage commitment in these same cells. Notch1 has been shown to signal through two distinct pathways: one CBF1-dependent and one deltex mediated. The experiments outlined will determine 1) Which pathway is engaged when Notch1 provides protection to T cell lines, and whether NFkappa.B is activated. 2) Which part of the Notch1 protein is required for mediating protection. 3) Which signaling pathway endogenous Notch 1 uses to mediate protection of thymocytes in vivo. 4) Whether Ras modulates endogenous Notch1 expression and activity when thymocytes are protected from apoptosis. We expect the data generated to considerably advance our understanding of the mechanism by which Notch1 mediates resistance to apoptosis in T lymphocytes.